E.P. George first pioneered the use of cosmic rays to measure the interior of a large structure in 1955, when he used the attenuation of cosmic rays to determine the overburden of the Guthega-Munyang tunnel in New South Wales, Australia. In the late 1960s, Luis Alvarez et al. then used measurements of the attenuation of cosmic ray muons to search for chambers within Chephren's pyramid. In response to post 9/11 concerns that terrorists might attempt to smuggle illicit nuclear materials or bombs via ports of entry, groups at Los Alamos National Laboratory and Uppsala University in Sweden have explored the use of cosmic ray muon tomography to detect Special Nuclear Materials (SNM) hidden within cargo containers.